1. Field of the Invention
The present invention relates to a technique for doping silicon with boron.
2. Description of the Prior Art
A variety of techniques have been utilized to dope semiconductor material. These may rely on a gaseous or solid source of dopant contacting the surface of the material. For example, it is known to diffuse a dopant from a vapor source so as to form a glass surface on a semiconductor body. The glass is then etched away, leaving a diffused surface region of the dopant in the semiconductor. Then, a heating step drives the dopant deeper into the semiconductor body a desired amount; see U.S. Pat. No. 3,066,052 coassigned with the present invention.
More recently ion implantation has been utilized, wherein ions are implanted into a surface region of a semiconductor body. The ions may be subsequently diffused further into the semiconductor body by a heating step, also referred to as a "drive-in" step. It is also known to implant the dopant into a solid material (e.g., polysilicon) contacting the surface of the semiconductor body, and subsequently diffuse the dopants into the semiconductor by a heating step, which also oxidizes the polysilicon; see U.S. Pat. No. 4,472,212 coassigned with the present invention. As a physical effect, it is known that the diffusion rate of common dopants (B, P, As) increases in silicon in the presence of an oxidation ambient; see "The Oxidation Rate Dependence of Oxidation - Enhanced Diffusion of Boron and Phosphorus in Silicon", A. M. R. Lin et al, Journal of the Electrochemical Society, p. 1131 (1981).
Recently, interest in dopant surfaces that are substantially vertical to the major surface of a semiconductor substrate has increased. This is due to a variety of proposals for forming "trenches" for isolation purposes, and to form vertical storage capacitors for dynamic random access memories (DRAM's); see, for example, "A Corrugated Capacitor Cell", H. Sunami et al, IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, pp. 746-753 (1984). However, problems have arisen when applying the traditional doping techniques to trench technology. Firstly, ion implantation per se is not feasible in most cases, since the directionality of an ion beam implies that the dopant will be implanted for the most part only in horizontal surfaces, as referenced to the semiconductor substrate major surface. This is because vertical surfaces will be substantially parallel to the ion beam direction, and hence will be shielded from the ion species by the overlying regions for the most part.
One method for doping trench sidewalls is to fill the trench with a glass that contains the desired dopant, and then to diffuse the dopant into the trench sidewall by a heating step. However, it is then frequently desirable to subsequently remove the glass dopant source, which in turn may create additional processing difficulties. The use of a gas dopant source has also been considered. However, in the case of the most common p-type dopant, boron, the solubility of the boron in silicon is so high that the doping is difficult to control. For example, a dopant concentration in the range of about 1.times.10.sup.16 to 5.times.10.sup.17 atoms/cm.sup.3 is typically desired for the p-type layer in trench capacitor sidewalls. The exposure of the sidewall to a boron-containing gas usually results in a doping level at the sidewall surface of about 1.times.10.sup.19 atoms/cm.sup.3. Even if a subsequent drive-in step is used to redistribute the boron deeper into the semiconductor, the doping concentration typically remains too high. Furthermore, the initial surface concentration is difficult to control.
It is also known to incorporate a dopant (e.g., arsenic) into a polysilicon layer, and diffuse the dopant into a silicion body by oxidizing the polysilicon; see U.S. Pat. No. 4,472,212 coassigned herewith. This technique utilizes the segregation effect, wherein the arsenic tends to be rejected from the moving silicon dioxide/silicon interface toward the silicon body. However, as applied to boron doping, this technique has the disadvantage that boron increases the difficulty of subsequently removing the oxidized polysilicon layer from the silicon body after the diffusion step. Hence, it is desirable to have an improved method for obtaining a boron-doped layer in a silicon body, including those with vertical features.